This invention relates to a rotary actuator using ball bearings and more particularly to this type of rotary actuator suitable for use in a rotary disc storage device.
JP-A-60-102862 published on June 7, 1985 discloses a rotary actuator comprising bearing sets each having a plurality of ball bearings. The shaft structure of the rotary actuator can be simplified and manufactured at low costs by using the rotary actuator, the rotational shaft of which is supported at its upper and lower ends by a ball bearing as in the above publication. But the rigidity of the single ball bearing at the both ends is small in the radial direction, resulting in a low mechanical resonance frequency of a rotary part of the rotary actuator. Consequently, vibrations are liable to be induced in the servo control process to degrade accuracy of positioning of a magnetic head on a magnetic disc. In another rotary actuator, its rotational shaft is supported at upper and lower ends by needle bearings for the purpose of improving radial rigidity. Since the radial rigidity of the needle bearing is large, a rotary part of the rotary actuator can have a high mechanical resonance frequency and is not liable to induce vibrations in the servo control. Disadvantageously, however, this rotational shaft has a complicated shape and requires additional thrust bearings, thus leading to higher costs.